Stabilization of liquid sulfur trioxide and oleums



United States P f STABELIZATIGN F LIQUID SULFUR TRIOXIDE AND OLEUMS George A. Peirce, Westiield, N. L, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del, a corporation of Delaware No Drawing. Application January 3, 1956 Serial No. 556,815

3 Claims. .(Cl. 23-1,74)

This invention relates to liquid sulfur trioxide and high strength oleums that are stabilized against polymerization; and to the stabilization thereof employing dimethyl sulfoxide as a polymerization inhibitor.

As is well known, sulfur trioxide exists in three forms having melting points of 17 C., 32.5 C. and 62 C. The polymeric form melting at 62 G, alpha S0 is the stable form under ordinary conditions. Upon freezing or even after standing at room temperature a short time the liquid gamma form (M. P. 17 C.) ordinarily changes to the solid forms.

This transition of liquid sulfur trioxide to solid polymeric forms is extremely undesirable from the standpoint of ease in handling and usage. In most instances, prior to usage the solidified sulfur trioxide must be remelted by heating to temperatures up to about 100 C., thereby developing dangerous high pressures in some instances.

High strength oleums of free sulfur trioxide strengths upward of about 80% tend to polymerize in a similar fashion. The degree of polymerization depends chiefly upon the sulfur trioxide strength of the oleum and the temperatures at which the material is stored. While polymerization will not proceed sufiiciently far in some of these oleurns to give a completely solid product, the degree of polymerization increases on the passage of time so that the ultimate mass will ordinarily contain solid forms of S0 in suspension in the oleum. This polymerization of sulfur trioxide in high strength oleums is also highly undesirable.

I have discovered that liquid sulfur trioxide and oleum of S0 strength such that S0 polymers tend to form can be stabilized against S0 polymerization by incorporating therein a minor amount of dimethyl sulfoxide.

The amount of dimethyl sulfoxide to be incorporated in the 50;, or oleum will vary with the use for which the SO -containing product is intended and with the conditions it will be expected to meet. The stabilizing influence of dimethyl sulfoxide is proportional to the amount present. In general, it usually is desirable to use only sufiicient amount to stabilize the product against substantial polymerization of S0 although relatively larger amounts of dimethyl sulfoxide can be used where the presence of an excess is not objectionable. In most cases it is desirable to use no more than about 10% of dimethyl sulfoxide, with from 0.2-5% based on the free 80;; content being the preferred range. The higher percentages ordinarily are employed to stabilize the oleums.

The preferred compositions of this invention consist of liquid sulfur trioxide or oleum containing not more than about 0.1% of water, which materials are stabilized with from 0.2 to 5% by Weight of dimethyl sulfoxide. Oleum containing not more than about 0.1% of water is, of course, oleum of S0 strength not less than about 99.4% and sulfuric acid content of not more than 0.56%. These materials high in sulfur trioxide can be stabilized readily so as to resist polymerization even after storage for long periods of time.

The dimethyl sulfoxide can be added directly to the Fhtenteddan. 21, 195;

V 2 liquid sulfur trioxide or oleum to be stabilized, followe by thorough agitation of the liquid. Alternatively, in tl case of stabilizing pure liquid sulfur trioxide, S0 vapr can be condensed in a closed vessel containing the desire amount of dimethyl sulfoxide.

After the dimethyl sulfoxide has been incorporated i the liquid sulfur trioxide or oleum, the stability of th product can be improved by heat-treating within the rang of about 50 to 100 C. for several hours. The preferre heat treatment is for at least 5 hours at about C.

Materials stabilized in accordance with my method d not polymerize to any appreciable extent. They remai substantially in the liquid gamma form at room tempera ture. Furthermore, the stabilization inhibitor functions 2 an antifreeze, lowering the freezing point substantial] in most instances. After freezing by being cooled to low temperature, these stabilized materials melt rapidl upon being reheated above about 25 C. Therefore, thes products can be used directcly in commercial applicatio without first putting them through a tedious remeltin process.

In order that the invention can be better understoot the following examples in addition to those set forth abov are given:

Example 1 0.5% by weight of dimethyl sulfoxide is added anhydrous liquid sulfur trioxide and the mixture is we] agitated. The liquid sulfur trioxide is then heated to five hours at 80 C.

The thusly stabilized sulfur trioxide is then cooled t1 5 C., at which temperature it shows no appreciabl tendency to solidify. Apparently, the addition of til dimethyl sulfoxide imparts a definite antifreeze action since gamma liquid sulfur trioxide ordinarily freezes a 17 C. Upon further cooling, it is found that the sta bilized sulfur trioxide freezes at -l5 C. Upon reheat ing to room temperature, the stabilized liquid sulfur tri oxide remelts rapidly. After standing at room tempera ture for several days, the composition exhibits no notice able polymerization.

Example 2 One percent by weight of dimethyl sulfoxide is adde( to anhydrous liquid sulfur trioxide and the mixture i: agitated. After standing at room temperature for 72 hours, the thusly stabilized liquid sulfur trioxide show: no tendency to polymerize and solidify. This stabilizer liquid sulfur trioxide is found to freeze on cooling to 0 C.; it remelts rapidly when reheated to room temperature Example 3 1.5% by weight of dimethyl sulfoxide is added with agitation to oleum of 80;, strength of 99.5%. After stand ing at room temperature for 72 hours, the thusly stabilized oleum shows no tendency towards S0 polymerization and solidification.

Example 4 Five percent of dimethyl sulfoxide is added to oleurr of 99.7% S0 strength. Upon standing for 72 hours at room temperature, the thusly stabilized oleum shows nc tendency towards solidification.

I claim:

1. The method of stabilizing against S0 p'olyrneriza tion a compound selected from the group consisting o1 liquid sulfur trioxide and oleum of S0 strength such that 80;, polymers tend to form comprising incorporating therein about 0.2 to 5% by weight based on 50;, content of dimethyl sulfoxide.

2. The method of stabilizing liquid sulfur trioxide containing not more than 0.1% of water against S0 poly- .erization comprising incorporatingtherein about 0.2 to 'by weight based on S0 content of dimethyl sulfoxide. 3. The method of stabilizing liquid sulfur trioxide gainst 7 S0 polymerization comprising incorporating lerein' about 0.2 to 5% by weight based on $0 content E dimethyl sulfoxide. a

4. The method of stabilizing against'SO polymeriza- 311 a compound selected from the group consisting of uid sulfur trioxide and oleum of S0 strength such lat S0 polymers tend to form comprising incorporating lerein about 0.2.to 5% by weight based on S0 content 5 dimethyl 's'ulfoxide and then heating for several hours a temperature in the; range of 50? C. to 100 C.

5, A,con'rp ositioncontaining a material selected from Le rou 0 1$ n 5Qf 1 1 rio i ean l um 3 rength such that-S0 polymers tend to form, in admixlre with dimethyl sulfoxide in an amount up to 10% y weight based on $0 content. 6. A compositionicomprising a materialselected fro the group consisting of sulfur trioxide and oleum of SO; strength such that S0 polymers tend to form, and about 0.2 to 10% by weight based on S0 content of dimethyl sulfoxide.

7. A composition comprising liquid sulfur trioxide containing not more than 0.1% of water and between about 0.2 and 5% by weightbased on S0 content of dimethyl sulfoxide.

8. The method of stabilizing against S0 polymerization the material selected from the group consisting of liquid sulfur trioxide and oleum of S0 strength such that $0 polymers tend to form comprising incorporating therein dimethyl sulfoxide in an amount up to 10% by weight based on S0 content.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF STABILIZING AGAINST SO3 POLYMERIZATION A COMPOUND SELECTED FROM THE GROUP CONSISTING OF LIQUID SULFUR TRIOXIDE AND OLEUM OF SO3 STRENGTH SUCH THAT SO3 POLYMERS TEND TO FORM COMPRISING INCORPORATING THEREIN ABOUT 0.2 TO 5% BY WEIGHT BASED ON SO3 CONTENT OF DIMETHYL SULFOXIDE. 